Imported a figure. Inserted and rearrranged bones from excisting character. Assiging the right groups to the bones. But with changing some body angles not all body parts "follow". For instance left forearm. All transform parameters work good, except bending. Some polygons do not follow and are stretched like a gummy band. Tried everything. Welding. Changing bones. Deleting hand. All left arm polygons are selected in the left forearm group. If I do Side-side first (no problem) and then Bend, results in the same problem. In other words, I cannot bend properly. In Setup room, the bones do respond as they should.

What am I overlooking.

Are your fall off zones excluding some polygons that should be included?

These tutorials may help:-

http://youtube.com/watch?v=PfcPu83gZNs

http://youtube.com/watch?v=434MJDE4gEA

I think what you are experiencing will be illustrated half way through the second video.

 I'm a little puzzled at the "all left arm polygons are selected in the LeftForearm group"

Do you mean that literally? If so, then there's your problem. 

To be more specific, all polygons from left elbow to wrist are in the "Leftforearm group". After inserting and modyfying the bones for the whole body, I used "Auto group" from the group editor and modified it a bit, because there were a few mismatches. Did the same with left  collar for instance and this works perfect.

It sounds like fall-off zone problems,IMO.  Philc is the master of fall-off zone problems.  If the inner or outer mat zones are off you'll have problems just exactly as you describe. You ought to check out his tutorials.

Yes, that's my problem. I can solve it now. Thanks all.

I didn't see the response from FhilC. I received a mail for a response in my thread and the link pointed direct at the reply from TrekkieGrrrl. But now I got it.